254 characters. From http://www.dominicsayers.com/isemail/ http://www.dominicsayers.com/isemail/: "Myth: The maximum length of an email address is 320 charact

254 characters. From http://www.dominicsayers.com/isemail/ http://www.dominicsayers.com/isemail/: "Myth: The maximum length of an email address is 320 characters This arises from the simple arithmetic of maximum length of a domain (255 characters) + maximum length of a mailbox (64 characters) + the @ symbol = 320 characters. Wrong. This canard is actually documented in the original version of RFC3696. It was corrected in the errata, but nobody reads the errata (except me it would appear). There's actually a restriction from RFC 5321 on the path element of an SMTP transaction of 256 characters. But this includes angled brackets around the email address, so the maximum length of an email address is 254 characters. You read it here first. This is not a new restriction - it goes right the way back to RFC 821 and was also in RFC 2821."